1. Field of the Invention
This invention relates to insulating bushings and more particularly to an insulating bushing in a high voltage system which raises the corona inception voltage and thereby avoids glow discharges at the junction of the insulator bushing and the metallic wall of some sort of electrical device.
2. Description of the Prior Art
In the art, ceramic bushings have been used to provide an electrically insulated entrance and exit for connections to electrical devices. The ceramic insulating body has a center conductor and a mounting wall fastened to it. These ceramic bushings have often been the source of corona in high voltage systems. Corona is the discharge of electricity appearing as a bluish-purple glow on the surface of and adjacent to a conductor when the voltage gradient exceeds a certain critical value. The corona is due to the ionization of the surrounding air by the high voltage. A detailed discussion on high voltage corona in bushings is given in Gaseous Conductors, written by James D. Cobine, 1958. In the prior art, a layer of electrically conductive material was placed between the insulating bushing and its retaining mounting, thus reducing the formation of flow discharges about the insulating bushing. Such a prior art system can be seen in U.S. Pat. No. 1,730,136 entitled, "Insulating Bushing," filed Feb. 27, 1926, by A. Haller.
Bell Telephone Laboratories has recognized that the mounting wall-bushing body area is critical when dealing with corona. The Bell Laboratories have attempted to raise the corona inception voltage of the ceramic bushing by shielding the air near the mounting area by an internal metal film and shielding the top of the bushing.
Corona inception voltage varies with the structural contour. At a sharp edge, such as the junction of the bushing and wall, the corona inception voltage is low. The prior art shielded such edges.
The apparatus of the invention eliminates the sharp metallic edges of the bushing, therefore, substantially increasing the corona inception voltage.